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TIFF images and alpha

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TIFF images and alpha Claus B. Madsen 27 Jan 10:29
Claus B. Madsen
2003-01-27 10:29:05 UTC (over 18 years ago)

TIFF images and alpha


I have a problem with The GIMP ... it is performins some undesired scaling of my RGB values when I change alpha value.

The situation is as follows. I have an application which can generate TIFF images. If I use this program to generate an image of some color, say RGB = [20, 30, 40] for all pixels, and set alpha = 255 for all pixels, and then load the generated image in GIMP, then everything is fine. Color picker says the color of all pixels is [20, 30, 40], and alpha 255.

But then if I generate a new image, exact same colors, but alpha = 128, and load the image in GIMP, then color picker says the color is [40, 60, 80], i.e., double the real values (although GIMP does tell me the right alpha value, 128).

I might add that loading the image with 'xv' tells me that the color in fact is [20, 30, 40], even though 'xv' cannot tell me what the alpha value is.

If I generate another image, same color values, but e.g. alpha 10, then color picker in GIMP says, alpha = 10, but by now the color values have been scales so much, that the colorbalance is not the same anymore, e.g., RGB = [237, 245, 255]. In this last example I am unsure about the exact result values (I was testing last night on my machine at home, and don't remember the exact values).

In summary: it seems GIMP is scaling the RGB values in response to alpha, alpha = 128 leads to double RGB values, alpha = 64 leads to four times the actual RGB values, etc.

Have I misunderstood something?